There has been well known such a picture quality adjusting circuit that upon transmitting a video signal, a correcting signal provided by secondary-differentiating an input video signal was superimposed upon the input video signal so as to improve the sharpness of a picture. However, when such a video signal in which a video signal with a large amplitude is superimposed thereon with a correcting signal having a relatively large level is produced from the conventional picture quality adjusting circuit, and a Braun-tube is driven by this signal, such a problem is brought about that the spot size of the electron beam is enlarged by a white peak with a value larger than a specified value and fine portions of the picture are damaged to cause white blurs thus resulting in picture degradation.
In order to prevent the picture quality from being deteriorated by the spot size of the electron beam enlarged, there may be considered two methods. One method is such that the white peak of the video signal applied to the Braun-tube can be detected and when the white peak is detected, the amplitude of the video signal is made low. Such circuit is called a peak ABL (automatic beam current limiter), in which a video signal S.sub.0 shown in FIG. 1A is converted into a video signal S.sub.0 ' shown in FIG. 1B. The other method is such that the white peak of a video signal can be similarly detected and when the white peak of the video signal shown in FIG. 2A is detected, the white side thereof is clipped or compressed thereby converted into a video signal S.sub.0 ' shown in FIG. 2B.
But, the former method has such problems that the circuit arrangement thereof is made complex, and when the white peak is detected, the contrast of the picture is lost suddenly. Whereas, the latter method inherits such a defect that as will be clear from FIG. 2B, the information on the white portion is likely to be lost.
An object of the present invention is to provide a picture quality adjusting circuit which can avoid white blurs caused by the spot size of the electron beam being enlarged.